The present invention relates generally to the field of aircraft rotors, and in particular to a rotor design for use in a helicopter or similar aircraft.
Helicopters generally incorporate at least two rotors into their design. The large rotor providing thrust in the vertical direction is known as the main rotor. In addition to this main rotor, the traditional helicopter design incorporates a tail rotor system to counteract the torque from the main rotor system. Although operable helicopter designs have been produced without the traditional tail rotor geometry, the vast majority of helicopters use this design. The number of blades in the tail rotor itself will depend on the requirements of a particular application.
A significant limitation inherent in the design of prior multi-bladed tail rotors is their inability to satisfactorily accommodate potentially powerful Coriolis torque. A Coriolis torque is generated in a helicopter rotor whenever the rotor plane is tilted relative to the shaft. Since the 1/rev Coriolis torque is proportional to the coning angle, it is usually negligible for most tail rotors. For a two-bladed tail rotor, the 2/rev Coriolis torque is also not a problem because both blades speed up and slow down at the same time, and the drive system is generally sufficiently flexible to provide the necessary torsional freedom. The 2/rev Coriolis torque does, however, become a problem with a multi-bladed rotor if insufficient lead-lag articulation is provided.
Existing multi-bladed tail rotors use a variety of methods to provide the necessary relief for 2/rev Coriolis torque. One design, developed by Sikorsky, uses a fully articulated rotor, complete with lead-lag hinges and dampers. Another design incorporates a flexible spindle at the blade root combined with restricted flapping motion to limit stresses due to Coriolis torque. One design, used by Kaman, allows a small amount of lead-lag motion by using a xe2x80x9crocking pinxe2x80x9d arrangement in its flapping hinge. Yet another design, developed by Lockheed, uses a gimbaled tail rotor hub that relieves the 2/rev Coriolis torque in the same manner as a two-bladed teetering rotor.
All of these designs suffer from limitations. In general, each of the above solutions is heavy and complex. Each requires the use of heavily-loaded bearings oscillating at tail rotor frequency, resulting in designs requiring high levels of maintenance and excessive downtime.
Accordingly, there is a need in the art for a tail rotor assembly overcoming the above-described limitations of the prior art designs, including reduction of tail rotor weight and mechanical complexity, reduction or elimination of catastrophic failure modes, and increased service life of the tail rotor mechanisms.
The following summary of the invention is provided to facilitate an understanding of some of the innovative features unique to the present invention, and is not intended to be a full description. A full appreciation of the various aspects of the invention can be gained by taking the entire specification, claims, drawings, and abstract as a whole.
The present invention relates to a dual-trunnion hub-to-mast assembly that provides improved damage tolerance with extended life expectancy and reduced maintenance burden due to the use of composite and elastomeric materials. In certain embodiments, the assembly is useful as part of a tail rotor assembly consisting of two stacked two-bladed teetering rotors, mounted on a single output shaft.
The present invention makes use of a variety of novel features to overcome the inherent limitations of the prior art. In certain embodiments, the present invention achieves increased-service life of the tail rotor mechanisms. In certain embodiments, the present invention achieves a reduction or elimination of catastrophic failure modes by the incorporation of redundant load paths within the rotor structure. In certain embodiments, the tail rotor of the present invention may be employed in a xe2x80x9cpusherxe2x80x9d implementation for improved aerodynamic performance by minimizing vertical fin blockage effects.
In addition to the above advantages, in certain embodiments the teachings of the present invention may provide improved aerodynamic efficiency, higher maneuvering capability, improved mechanical flaw tolerance design, and extended life expectancy. In certain embodiments, the present invention allows for reduced maintenance due to the use of composites and elastomerics. In one embodiment, a tail rotor constructed according to the present invention has been designed to achieve a minimum life of 10,000 hours of severe duty use in ground-air-ground maneuvers, air combat maneuvers, and high cycle vibratory loads, with little or no maintenance.
In certain embodiments, the present invention makes extensive use of multiple primary load paths in order to provide a fail-safe structure. In certain embodiments, the present invention provides redundant load paths for critical metal parts to minimize catastrophic failure modes. Certain embodiments eliminate the use of the bearings traditionally required to carry the full centrifugal force of the blade while oscillating at tail rotor one-per-revolution. This is done in order to further increase life expectancy, improve reliability, and minimize maintenance. In certain embodiments, the present invention minimizes control washout to the blades due to control system softness.
As described above, a significant limitation inherent in the design of prior multi-bladed tail rotors is their inability to satisfactorily accommodate potentially powerful Coriolis torque. A Coriolis torque is generated in a helicopter rotor whenever the rotor plane is tilted relative to the shaft. Since the 1/rev Coriolis torque is proportional to the coning angle, it is usually negligible for most tail rotors. For a two-bladed tail rotor, the 2/rev Coriolis torque is also not a problem because both blades speed up and slow down at the same time, and the drive system is generally sufficiently flexible to provide the necessary torsional freedom. The 2/rev Coriolis torque does, however, become a problem with a multi-bladed rotor if insufficient lead-lag articulation is provided.
Existing multi-bladed tail rotors use a variety of methods to provide the necessary relief for 2/rev Coriolis torque. One design, developed by Sikorsky, uses a fully articulated rotor, complete with lead-lag hinges and dampers. Another design incorporates a flexible spindle at the blade root combined with restricted flapping motion to limit stresses due to Coriolis torque. Another design, used by Kaman, allows a small amount of lead-lag motion by using a xe2x80x9crocking pinxe2x80x9d arrangement in its flapping hinge. Yet another design, developed by Lockheed, uses a gimbaled tail rotor hub that relieves the 2/rev Coriolis torque in the same manner as a two-bladed teetering rotor.
All of these designs suffer from inherent limitations. In general, each of the above solutions is heavy and complex. Each requires the use of highly-loaded bearings oscillating at tail rotor frequency, resulting in designs requiring high levels of maintenance and excessive downtime.
One manner of addressing this problem is to mount a pair of two-bladed rotors on the same shaft. This arrangement provides a four-bladed tail rotor with the mechanical and structural simplicity of a two-bladed teetering rotor. By using this concept, no bearings are required to oscillate while carrying the full centrifugal force of the blade.
Although this solution partially addresses the above-described problems, it does not inherently provide relief for the 2/rev Coriolis torque. With this design, whenever the tail rotor experiences first harmonic flapping, one pair of blades will be attempting to accelerate at the same instant in time that the other pair of blades is attempting to decelerate. Thus, the two rotors will try to move in the same manner as a pair of scissors, placing considerable stress on the rotor hub components.
In spite of these limitations, variations on this approach have been employed successfully in aircraft. One design uses a double-teetering tail-rotor with coaxial shafts. Aircraft using this design have been successfully flown. Another design uses a double-teetering tail rotor with flexible forks. While both these approaches provide the desired relief for 2/rev Coriolis torque, there are several disadvantages associated with each one. The designs exhibit increased mechanical complexity and a heavier design. In addition, there are problems associated with tailoring the stiffness of critical metal parts, possibly resulting in a degraded structural design and potentially catastrophic failure modes.
The tail rotor of the present invention utilizes a modification of the above approach. In one embodiment of the present invention, each of a pair of two-bladed rotor assemblies is independently mounted on a common drive shaft. Each rotor assembly is a two-bladed teetering rotor. The spanwise axis of the blade-pair units are perpendicular to each other, and are separated axially to provide adequate space for accommodating hub attachment hardware and operational clearance between them.
The 2/rev Coriolis relief for the tail rotor system of the present invention is provided by optimizing the dynamic charactericstics of an existing component in the system rather than by adding additional hardware. The rotor assembly uses an elastomeric bearing to accommodate rotor flapping. Conventional teetering rotors that use elastomeric bearings to provide flapping degree of freedom require the radial stiffness of the bearings to be very high in order to minimize radial deflection under rotor torque. In the design of the present invention, however, the bearing radial stiffness is tailored to provide adequate stiffness to react rotor torque and to provide adequate softness to relieve for 2/rev Coriolis loads. Since the Coriolis relief is provided by tailoring the spring rate of an existing component required to accommodate the xe2x80x9cflappingxe2x80x9d degree of freedom anyway, the resulting hub assembly provides a much simpler configuration with reduced weight and cost, and higher reliability due to reduction in number of parts.
In one embodiment of the present invention, an inboard trunnion and outboard trunnion are clamped together on a tail rotor mast using a pair of tapered cones, a hub adapter, and a mast nut. The tail rotor mast transmits drive torque to the inboard trunnion by means of a spline section. The inboard trunnion has mating splines on its inside surface to mate with the mast spline section, and curvic teeth on its outboard face to mate with the corresponding teeth on the hub adapter.
A cone set between the inboard trunnion and inboard shoulder of the mast provides positive centering of the inboard trunnion and locks out radial looseness in the spline section. The drive torque is transmitted to the outboard trunnion from the inboard trunnion through a hub adapter having an inboard curvic coupling mating with the inboard trunnion and an outboard curvic coupling mating with the outboard trunnion. A second cone set between the outboard trunnion and the mast nut provides centering of the outboard trunnion.
In certain embodiments, the section of the mast outboard of the inboard trunnion has a reduced outside diameter to produce a torsional stiffness significantly lower than the torsional stiffness of the hub adapter. Thus for any rotational deflection of the outboard trunnion, the mast will rotate an equivalent amount with this rotation occurring in the reduced section of the mast and not at the outboard cone set joint.
In certain embodiments, the cones, trunnions, and hub adapter slide over the tail rotor mast and are sandwiched between an integral shoulder of the mast and the mast nut. The mast nut torque produces an axial pre-load across these components. The axial pre-load generates the desirable frictional clamp up at the outboard cone and counteracts separation force from the curvic coupling joints.
The primary purpose of the hub adapter is to deliver drive torque to the outboard trunnion. The inboard trunnion is splined to the mast. Accordingly, all of the steady drive torque from the mast goes into the inboard trunnion. Approximately half of that torque goes into the inboard rotor through the inboard yoke. The remaining drive torque exits the inboard trunnion, goes through the hub adapter, and into the outboard trunnion, which drives the outboard yoke and outboard blades. It will be noted that the outboard trunnion is not splined to the mast. Therefore the hub adapter sees about one half of the mast torque as a steady load.
The above-described structure provides a number of benefits, including reduced failure due to fretting and wear, the absence of relative motion at the attachment joints, and commonality between the inboard and outboard rotor assemblies. Since the 2/rev Coriolis torque loads between the inboard trunnion and outboard trunnion are reacted by the curvic couplings, and not the tail rotor mast spline section, the potential failure due to fretting is reduced. Since the two stacked rotor trunnions are clamped together through curvic couplings, they are securely fixed to one another via a tight joint, which is desirable for minimizing the fretting and wear common to joints that see high oscillatory loads. The torsionally-soft outboard section of the mast accommodates the angular deflection between the two trunnions with minimal relative motion occurring at the attachment joint surfaces. Finally, this design allows for common inboard and outboard rotor assemblies, which can be assembled, replaced and shipped as individual 2-bladed assemblies.
There are at least two major design considerations in the sizing of the curvic couplings of the hub adapter. First, each of the couplings must be capable of reacting the steady, oscillatory, and limit torque loads imposed by the tail rotor. Second, it is desirable that the axial pre-load across the couplings be high enough to prevent joint separation during operation. The couplings and surrounding hardware (cone sets, mast nut, and mast) must also be capable of carrying the pre-load requirement. The size and pitch of the curvic couplings will, of course, vary from one application to another.
The novel features of the present invention will become apparent to those of skill in the art upon examination of the following detailed description of the invention. It should be understood, however, that the detailed description of the invention and the specific examples presented, while indicating certain embodiments of the present invention, are provided for illustration purposes only because various changes and modifications within the spirit and scope of the invention will become apparent to those of skill in the art from the detailed description of the invention and claims that follow.